PCB planar lead soldering often involves limited operating space around the pad. Under such conditions, conventional contact-based soldering processes are prone to interference with other components and may introduce additional stress. To address this issue, this paper proposes a high-temperature molten solder droplet jetting soldering method based on the water hammer effect. In this method, a piezoelectric ceramic actuator drives the jetting tube in high-frequency reciprocating motion, generating transient pressure at the orifice to achieve on-demand ejection and non-contact deposition of molten solder droplets, thereby enabling printed circuit board lead soldering. The dynamic processes of droplet flight, coalescence, and spreading were observed using a high-speed camera, and the transient behavior of the jetting soldering process was clarified. The effects of key process parameters on solder joint formation characteristics were systematically investigated. It was found that the macroscopic morphology, wetting state, and intermetallic compound layer of the solder joints are jointly regulated by multiple factors, which in turn affect their mechanical properties. Shear tests showed that the mechanical characteristics of the solder joints are closely related to their morphology and interfacial microstructure. This method can complete soldering within 0.1s and features high efficiency, non-contact operation, and strong controllability. This study not only verifies the feasibility of water-hammer-based molten solder droplet jetting soldering for PCB lead soldering, but also provides a new scientific basis for precision soldering under space-constrained conditions.

Aluminum alloy materials, due to their low density, high strength, and corrosion resistance, have been widely used in various fields. Currently, various industries place great importance on the manufacturing quality of aluminum alloy mechanical structural components, striving for lightweight designs while ensuring reliability. Since the production of certain components involves welding technology, the requirements for the welding quality of aluminum alloy parts are steadily increasing. The quality of aluminum alloy welding directly impacts the practical application of welded parts, while the welding test research and evaluation system can measure the efficiency and quality of aluminum alloy welding. This paper reviews the experimental and testing methods used in recent years for aluminum alloy welding research, points out the shortcomings and areas for improvement in the experimental and testing approaches currently used in welding research, and summarizes and proposes a more comprehensive evaluation system for aluminum alloy welding experimental testing.

This study addresses the limitations of conventional BGA (Ball grid array) solder ball placement processes—including low efficiency, structural complexity, and excessive reliance on templates—by proposing a piezoelectric jetting method based on the water-hammer effect for high-frequency solder bump formation. We comprehensively analyze the mechanism of molten solder droplet ejection through water-hammer actuation, with the droplet formation process visualized via high-speed photography. An in-depth analysis elucidates the fundamental principles governing droplet generation. Through optimized trajectory planning for the three-axis motion platform, high-quality solder ball bump arrays are successfully fabricated.

Satellite-droplet formation remains a fundamental obstacle to achieving high-resolution outcomes in piezoelectric drop-on-demand inkjet printing, a critical additive manufacturing technology for precise material deposition. This work elucidates the underlying acousto-fluidic coupling mechanisms governing satellite-droplet generation through integrated high-speed imaging and numerical simulation. By analyzing channel acoustics and interface dynamics, two active suppression strategies implemented solely through acoustic wave control are proposed: Strategy (a) operates by attenuating the acoustic pressure intensity in the printhead channel, which in turn reduces the excitation intensity of interface motions and ultimately promotes main- and satellite-droplet coalescence for satellite-droplet suppression. Strategy (b) operates by actively retracting the liquid-ligament via a secondary positive pressure wave before it fragments into satellite-droplet(s). Both strategies require no external fields or ink modifications, ensuring broad applicability across diverse materials and environmental conditions. Experimental validation demonstrates robust suppression of satellite-droplet, yielding well-defined and high-fidelity droplet arrays. This work provides a universal and acoustically driven route to highprecision droplet generation, which could significantly advance the accuracy and reliability of inkjet-based additive manufacturing technologies.

Unsupported inclined pillars serve as the foundation of lattice structures. However, during the fabrication of lowangle inclined pillars via metal droplet deposition manufacturing, droplets are prone to contact the substrate and cause collapse, which limits the capability of this technique for manufacturing lattice structures. This study investigates the collapse phenomenon of droplets during the fabrication of inclined pillars by metal droplet deposition manufacturing. Based on experimental and simulation results, the dynamic process of offset droplet deposition is analyzed. It is found that after contacting the preceding droplet, the motion of the offset droplet can be regarded as a combination of global motion and spreading-retraction motion. During this combined motion, the lowest point of the offset droplet first descends and then rises. If the droplet contacts the substrate during the descending stage, collapse occurs. Then influencing factors of droplet collapse are investigated, and three methods are proposed to prevent droplet-substrate contact: pre-fabricating a base, reducing droplet velocity, and increasing oxygen content. Among these, the first method proves to be the most effective, since it only requires depositing additional droplets beneath the bottommost droplet of the inclined pillar to form a base, without the need for any other modifications to the process parameters. This method enables the fabrication of inclined pillars with an angle of approximately 10 degrees. Furthermore, the influence of the lateral displacement of offset droplets during deposition on the inclination angle of the pillars is analyzed, and a self-stabilizing mechanism of the pillar inclination angle is proposed as an explanation. This study contributes to enhancing the capability of metal droplet deposition additive manufacturing for fabricating lattice structures.

The rapid development of electronics has driven integrated circuit packaging toward high-density and miniaturization. Ball Grid Array (BGA) packaging is currently a mature high-density packaging technology. BGA and & micro;BGA packaging impose strict requirements on the uniformity of solder balls. As packaging sizes continue to shrink, sub-100 & micro;m solder balls have become critical elements for high-density interconnects, with preparation difficulty increasing dramatically as size decreases. This study proposes a sub-100 & micro;m tin ball preparation technology using droplet-on-demand jetting. By utilizing a self-developed water-hammer droplet-on-demand jetting system, it achieves a 2000 Hz(2 kHz) ball-forming frequency and diameter deviations within +/- 3 & micro;m. Compared to traditional tin ball preparation methods, this approach demonstrates superior tin ball fabrication capabilities, establishing a new paradigm for efficient and precise manufacturing of micron-scale solder balls.

The electrohydrodynamic (EHD) inkjet printing is rooted in the creep effect exhibited by fluid interfaces subjected to high-intensity electric fields. It utilizes the micro-jetting from the tip of the Taylor cone to print intricate patterns with sub-micron feature sizes. However, due to the limitations inherent in the jetting mechanism, the highest printing frequencies for conventional EHD are limited to several hundred Hertz. Here, we discover a novel phenomenon where the vibration of meniscus in an electric field can induce EHD jetting within microseconds, or even sub-microseconds. Based on this phenomenon, we propose a new method for achieving megahertz level EHD jetting based on meniscus vibrations (MVEHD). This paper systematically elucidates the fundamental mechanisms of the forced meniscus vibration, cone sharpening, and the jetting formation through a combination of experimental research and numerical simulation. Additionally, adjustable continues inkjet (CIJ) and drop on demand (DOD) inkjet printing modes were developed, demonstrating the potential of dual-mode technology in the development of automotive transparent glass heaters (TGHs) and Micro-LED displays. This method effectively elevates the efficiency of traditional EHD by 2 to 4 orders of magnitude, thereby laying a theoretical and technical foundation for the industrial application of high-throughput EHD technology.

Current drop-on-demand metal jetting technologies provide precise control over the start and stop of jetting, as well as the exact number of droplets ejected. However, these methods are generally constrained by low jetting frequencies. Conversely, continuous jetting technologies can achieve significantly higher jetting frequencies but lack the capability to precisely regulate the start and stop of jetting, as well as the exact number of droplets. To address the limitations of existing metal droplet jetting techniques, this study introduces a novel high-frequency jet-on-demand jetting approach utilizing the forced vibration of the jet tube. By inducing high-frequency forced vibration along the axial direction of the jet tube, molten metal within the tube is ejected at the same frequency, facilitating precise and efficient droplet formation. The proposed jetting method achieves start times of less than 9 ms, significantly shorter than those of conventional continuous-flow jetting techniques controlled by back pressure. Furthermore, the harmonic characteristics of the forced vibration effectively eliminate residual oscillations from single excitations, enabling a significant enhancement in jetting frequency. Experimental results confirm that stable and uniform jetting can be achieved across a range of forced vibration frequencies, with jetting frequencies reaching up to 5 kHz. Forming experiments further validate the superiority of this method over conventional drop-on-demand jetting techniques, demonstrating significantly higher forming efficiency and producing metal components with enhanced mechanical properties.

Appropriate weld penetration is of vital significance for ensuring the welding quality of gas tungsten arc welding (GTAW). Visual monitoring based on deep learning has been widely applied in weld penetration monitoring. However, deep learning requires a large number of labeled samples to achieve satisfactory performance. Deep transfer learning (DTL) is an effective technique to address this issue, but the famous ImageNet dataset may not be suitable for pre-training a deep learning model for weld penetration prediction. In this study, a visual monitoring approach for weld penetration of aluminum alloy GTAW based on DTL enhanced by task-specific pre- training and semi-supervised learning (SSL) is proposed to obtain better prediction accuracy of the backside bead width with limited labeled data. Firstly, an active vision method is used to capture images of the weld pool. Next, a task-specific pre-training method is designed by constructing a keypoint localization task to pre-train a deep learning model with an encoder-decoder architecture, and SSL is introduced to reduce the required number of labeled data in pre-training. Finally, an encoder-based regression model is constructed and fine-tuned to predict the backside bead width. It is found that by using SSL in task-specific pre-training, the keypoint localization model trained with only 40 labeled samples can achieve ideal performance, and the performance of SSL outperforms fully-supervised learning (FSL) in terms of both keypoint localization accuracy and robustness to the randomness of labeled training samples. Moreover, the mean prediction error of backside bead width after finetuning is only 0.176 mm, which is reduced by 29.9 % compared to using ImageNet for pre-training. The proposed method also has good real-time performance and thus has the capability to be applied in the real-time monitoring and control of weld penetration.

Inkjet printing techniques are often used for bioprinting purposes because of their excellent printing characteristics, such as high cell viability and low apoptotic rate, contactlessmodus operandi, commercial availability, and low cost. However, they face some disadvantages, such as the use of bioinks of low viscosity, cell damage due to shear stress caused by drop ejection and jetting velocity, as well as a narrow range of available bioinks that still challenge the inkjet printing technology. New technological solutions are required to overcome these obstacles. Pneumatic conveying printing, a new type of inkjet-based printing technique, was applied for the bioprinting of both acellular and cellular fibrin-hydrogel droplets. Drops of a bioink containing 6 × 106HEK293H cells ml-1were supplied from a sterile nozzle connected to a syringe pump and deposited on a gas stream on a fibrinogen-coated glass slide, here referred to as biopaper. Fibrinogen film is the substrate of the polymerization reaction with thrombin and Ca2+present in the bioink. The pneumatic conveying printing technique operates on a mechanism by which drop ejection and deposition in a stream of gas occurs. The percentage of unprinted and printed dead HEK293H cells was 5 ± 2% and 7 ± 4%, respectively. Thus, compared to normal handling, pneumatic conveying printing causes only little damage to the cells. The velocity of the drop approaching the biopaper surface is below 0.2 m s-1and does not cause any damage to the cells. The cell viability of printed cells was 93%, being an excellent value for inkjet printing technology. The HEK293H cells exhibited approximately a 24 h lag time of proliferation that was preceded by intense migration and aggregation. Control experiments proved that the cell migration and lag time were associated with the chemical nature of the fibrin hydrogel and not with cell stress.

Realizing real-time monitoring of the machining state and product quality during the manufacturing process is essential for ensuring part quality and improving production efficiency. Machining state fluctuations often indicate production process instability, resulting in increased costs, including accelerated tool wear and degradation of workpiece quality. However, the complexity and time-varying nature of machining processes make it challenging for traditional methods to achieve real-time, accurate monitoring. This paper establishes a data-mechanism hybrid-driven digital twin system (DMH-DTS) framework through the internal real-time data of the machine tool to achieve high-precision, real-time visualization monitoring of machining states and quality without the need for any additional sensors. The composition and working principle of the DMH-DTS are elaborated in detail, and the key enabling technologies of the DMH-DTS are introduced. The motion model and environment model of the DTS, alongside the geometric model, material model, and behavior model of the material removal process are established from perspectives of the machining mechanism and data fusion, respectively. Through the mutual flow of energy, material, and information in the DMH-DTS, large-scale multi-source heterogeneous data fusion is realized, ensuring real-time symmetry between the physical and virtual systems. The cross-scale online visualization monitoring method of the grinding state proposed in this paper enables real-time monitoring of both the grinding state and the shape accuracy of the workpiece. The visual prediction of workpiece machining accuracy can be achieved in the crossscale range (mm, mu m, and nm). Furthermore, this method facilitates the traceability of machining errors, enabling the identification of the causes of workpiece errors, thereby providing a theoretical basis for further improving product accuracy and quality.

The molten metal drop-on-demand (DoD) jetting technology holds great potential for applications in fields such as electronics manufacturing and metal additive manufacturing. To ensure the stability and reliability of droplet quality, monitoring the jetting system state and controlling the jetting process are essential. However, existing studies primarily focus on conventional ink rather than molten metal, while monitoring the jetting system state and controlling the jetting process remain unintegrated. This study develops an integrated intelligent monitoring and control system for a self-developed water-hammer-based molten metal DoD jetting system. Firstly, the geometric features of droplets are collected and analyzed under varying liquid level heights and different driving waveform parameters. Subsequently, a jetting simulation model is established, and then it is used for sampling to train a prediction model for droplet volume reachable range, which can be used to monitor the state of the jetting system. Next, a multi-objective molten droplet control system is designed based on deep reinforcement learning, enabling the control system to simultaneously regulate the droplet volume and shape. Finally, simulations and experiments are conducted on the developed monitoring and control system. The results demonstrate that the monitoring system can accurately determine the current state of the liquid level. Furthermore, under normal liquid level height, the control system can achieve precise and rapid control over both the droplet volume and shape. This study contributes to improving the production quality of molten metal DoD jetting systems.

While high-energy beam-based laser additive manufacturing technology boasts high formability efficiency, its large equipment size and high manufacturing cost limit its application in certain scenarios. In contrast, the direct writing metal additive manufacturing approach showcases attributes of a compact heating apparatus, superior energy utilization, and the utilization of metal wire as raw material, culminating in notable reductions in manufacturing costs. However, conventional direct writing technology often faces issues of nozzle clogging, which adversely impacts the quality of the formed structures. This paper introduces an ultrasonic-assisted direct writing metal additive manufacturing (UADWMAM) technique that addresses the nozzle clogging problem by applying ultrasonic vibrations, thereby enhancing the stability of molten metal extrusion. Further investigations were conducted through single-pass forming experiments, examining the influence of process parameters, such as writing speed and heating temperature, on writing formability. The findings from this study have the potential to advance the development of direct writing metal additive manufacturing technology.

Utilizing the residual droplet produced from liquid bridge breakup for microdroplet deposition serves as an important supplementary method to conventional printing techniques. However, this approach typically relies on mechanical motion to form and break the liquid bridge between the liquid donor and acceptor surfaces, resulting in a relatively complex process and low printing efficiency (typically limited to several Hertz). Here, we propose a novel contact printing method based on the meniscus vibration (MVCP). A tubular piezoelectric dispenser is employed as the liquid donor, with the acceptor surface positioned at a distance of several tens of micrometers from the nozzle. By modulating the waveform of the driving signal, the meniscus can undergo controlled extrusion and withdrawal, enabling the precise formation and breakup of the liquid bridge. Experimental results indicate that MVCP offers advantages such as a simplified process, high printing frequency (several tens of Hertz), and droplet sizes smaller than the nozzle diameter. A combined approach of experimental research, numerical simulation, and mechanics analysis was used to systematically investigate the mechanisms of meniscus vibration, liquid bridge formation, and breakup. The findings indicate that the performance of the MVCP is strongly influenced by the hydrophilicity of the acceptor surface as well as the vibration amplitude of the meniscus. Additionally, an on-demand printing strategy for low-viscosity inks was developed, demonstrating MVCP's potential for high-resolution printing and providing a foundational basis for its further development and application.

Microfluidic technology plays a vital role in modern industry and advanced scientific research due to its exceptional features. Owing to the small scales and minor density differences between multiphase fluids, the impact of microgravity on the droplet behavior has been largely overlooked. However, this work reveals that even subtle density differences can have a butterfly effect on the behavior of microdroplet. Droplets within microchannels exhibit three distinct motion regimes: (i) contacting mode: the droplet gradually descends during its movement along the channel and finally contacts the wall; (ii) floating mode: the droplet descends first but floats out maintaining a stable distance from the wall; (iii) bouncing mode: the droplet bounces off the wall at the moment of contacting it. This study uncovers the physical mechanisms of how the working parameters influence droplet behavior and establishes a phase diagram depicting the droplet behavior.

On-demand droplet jetting can be achieved through the axial reciprocating motion of the jet tube, which is a novel technology. However, its feasibility for high-frequency jetting has not been explored. In this study, it is found that when high-frequency driving signals are applied to the piezoelectric ceramic, the tube undergoes forced vibration, which enables repeatable jetting at frequencies of up to 10 kHz. This paper also investigates the effects of the vibration amplitude and frequency of the tube on the jetting behavior and reveals a linear relationship between the jet velocity and the tube vibration velocity amplitude.

Amid the global wave of intelligentization, flexible pressure sensors have emerged as core sensing components owing to their excellent flexibility, portability, and highly sensitive response to pressure signals in fields such as human-computer interaction, health monitoring, smart wearable devices, and artificial-intelligence terminals. Herein, we developed a flexible pressure sensor with a hierarchically porous structure via a hybrid manufacturing strategy integrating three-dimensional (3D) printing and electrospinning. A MXene/silver nanowires/polydimethylsiloxane (MAP) conductive ink modified with nano silicon dioxide (SiO2) was formulated to optimize printability, and the multi-scale porous sensing unit was constructed by direct ink writing (DIW) combined with a sacrificial template method, followed by encapsulation with a fibrous film electrode. Benefiting from the synergistic effect of macro-micro porous structure and the structural-mechanical matching design, the sensor exhibits high sensitivity (0.813 kPa-1), fast response (37 ms/30 ms), and an extremely low detection limit (1.47 Pa), thus enabling the effective monitoring of various human physiological motion signals and demonstrating significant potential in health monitoring applications. Furthermore, by integrating deep learning algorithms, high-accuracy recognition (94.9 %) of several spoken phrases is achieved, extending the sensor's applicability to human-computer interaction and intelligent speech perception. In summary, this study proposes a strategy based on a hybrid manufacturing technology for fabricating hierarchically porous flexible pressure sensors, demonstrating broad application prospects in health monitoring and smart human-computer interaction, thus providing new pathways for the innovative design and functional expansion of wearable electronic devices.

This study addresses the drawbacks of existing piezoelectric inkjet dispenser, namely long production cycles and nozzle fragility, by developing a separable piezoelectric printing nozzle suitable for both inkjet and electrohydrodynamic (EHD) printing. When the nozzle is damaged or requires a different size, it can be replaced easily by plugging and unplugging, significantly reducing usage costs and production time. We used a 30% glycerol-water solution as the ink and tested the nozzle’s functionality in both continuous and on-demand modes for inkjet printing and EHD jetting, respectively. Functional patterns were printed to verify the practical printing capability of the fabricated nozzle. The working principle of the nozzle was detailed, revealing that the oscillation and jetting at the nozzle orifice are primarily caused by the transmission, reflection, and superposition of one-dimensional pressure waves within the tube.

Wetting of liquid droplets is a fundamental topic in interfacial science. Owing to their intrinsically high surface tension, liquid metals typically exhibit poor wettability on metallic substrates. Recent studies have shown that interfacial reactions between liquid metals and solid metal surfaces can promote wetting; however, such reactive wetting is generally irreversible because of the permanent chemical changes involved. In this work, we report a reversible wetting behavior of Galinstan droplets on copper substrates under alkaline conditions. Specifically, Galinstan undergoes interfacial redox reactions with metal oxides that possess standard reduction potentials higher than that of gallium, leading to the formation of Ga2Os and other oxides. This reaction significantly lowers the interfacial tension, thereby driving spontaneous spreading of the droplet. Meanwhile, hydroxide ions (OH-) in the alkaline solution can dissolve the interfacial Ga2Os layer, enabling retraction of the droplet and thus imparting reversibility to the wetting process. This finding introduces a new route for achieving dynamic and controllable wetting of liquid metals.